Infectious Disease Diagnostics Flashcards
what are the two classes of diagnostic tests
- pathogen detection
- host response to pathogen
pathogen detection diagnostics
detect the pathogen itself
- whole pathogen/culture
- nucleic acid based
- pathogen antigens
host response to pathogen diagnostics
detects aspects of the patient’s response to the pathogen
- antibody to the pathogen
- cell mediated response to the pathogen
culture
use of specialized media and culture conditions to isolate live and whole organisms from a specimen collected from the patient
what are the types of cultures
- agar media
- cell culture
- whole animal
agar media culture
bacteria or fungi
- not all organisms grow on agar
does NOT ID organism - need additional testing
cell culture
obligate intracellular bacteria and viruses
have to grow a cell line from same species as the patient
does NOT ID organism - need additional testing
whole animal culture
rarely done
used when pathogen cannot grow outside a living creature
when should culture be used
- when you need the organism to do further testing
- organisms that grow quickly in standard conditions
- discovering novel organisms
what does a + and - culture mean
+: pathogen present but does not always mean it is causing disease
-: does not rule out disease but makes it less likely
nucleic acid testing
PCR testing - detects DNA (or RNA converted to DNA)
when to use PCR
- slow growing/unculturable pathogens
- need rapid results (hours)
- acute infections
- biohazardous agents that you do not want to culture
- in conduction with culture to ID organisms
what are the downfalls of PCR
- have to know the DNA sequence
- cause only use certain sample types
- DNA and RNA can degrade - have to test quickly w/ adequate sample size
- may detect subclinical disease
what are PCR panels
tests for multiple organisms at once
- less expensive
- more difficult to interpret - not all pathogens detected may be causing disease
what does a + and - PCR test indicate
+: above threshold; piece of DNA of interest was detected in the patient sample
-: below threshold; pieces of DNA of interest was not detected in the patient sample
what causes false positive and negatives on PCR
false positive: previous infection, contamination
false negative: inhibitory substances in sample, lower amount of DNA than test sensitivity
antigen detection tests
detects one or more antigens on/from the pathogen
- IHC
- DFA
- lateral flow assays
- ELISA
what is required for an antigen detection test to work
- a known antibody that can detect antigen
- a way to visualize the antigen/antibody complex once formed (color, fluorescence)
steps of antigen ELISA tests
- antibody to the antigen is immobilized on substrate
- sample is processed and put on/in substrate
- second antibody is added to also recognizes bound antigen
- wash
- detection of second bound antibody
steps of IHC and DFA
- take a tissue section or smear containing suspected pathogen
- antibody is added to the sample with an enzyme or fluorescent tag
- color change or fluorescence occurs if antibody binds the antigen
IHC: color change; use with tissue biopsies
DFA: fluorescence; use with smears/stains
what does a + and - antigen detection test indicate
+: pathogen present but not always causing disease
-: does not rule out disease but makes it less likely
antibody detection tests
detects the presence of antibodies against a pathogen in patient’s sample
- ELISA
- IFA
- agglutination
- immunoblot
- AGID
when to use antibody detection tests
- chronic disease
- paired measurements/titers (acute sample + chronic sample)
- pathogens that are difficult to culture
benefits of antibody tests
- good for hard to culture pathogens
- paired tests can be used for acute disease
- detection of IgM can indicate acute exposure
downfalls of antibody detection tests
- not good for acute disease - takes time for patient to mount an antibody response
- may have low sensitivity
- false positives can occur with vaccination/subclinical disease
- positive results doesn’t always mean infection
- not good for immunocompromised animals
- may need multiple samples
what does a + and - antibody test indicate
+: indicates exposure or vaccination; not necessarily live infection
-: indicates no exposure/infection OR possible acute infection OR patient is immunocompromised
cell-mediated immune response tests
detects cell mediated cytokine responses
- skin tests (Tb)
not commonly done
what tests should generally be done for acute disease
pathogen detection
- culture
- antigen
what tests should generally be done for chronic disease
host response detection
- antibody
- cell mediated
what tests should generally be done for immunocompromised patients
pathogen detection
- culture
- antigen
what tests should generally be done for undetectable organisms
host response detection
- antibody
- cell mediated
what diagnostic test should be used for a cat with a large growth on its nose w/ suspected Cryptococcus
collect an aspirate of the mass for direct smear staining and microscopy
why might only 1 species of bacteria be on an aerobic culture if 2 species were visualized on direct gram stain
the second species may be a strict anaerobe
what is the best way to transport samples from a horse transtracheal wash and pleural fluid for culture
ship each in a sterile white top or red top with a cold pack overnight to the lab
what is the best diagnostic test for a horse with pleuropneumonia? samples are from transtracheal wash and pleural fluid
- gram stain
- aerobic and anaerobic culture
- susceptibility testing
what stain would be most beneficial for an FNA from a 7 yo indoor/outdoor cat with ulcerative, proliferative lesions on the paw that do not respond to antimicrobial therapy
acid fast stain
(can ID mycobacterium)
what is the best way to test for Francisella tularensis
PCR from a small piece of spleen
(do NOT culture due to Tularemia risk)
why might a 3 month old goat doe receive a positive CAE ELISA antibody assay
a 3 month old goat is young enough to still have circulating maternal antibodies, causing a false positive
what diagnostic test would you select for an 18 month old quarter horse with stumbling/incoordination, fever, muscle fasciculations, and ataxia
West Nile virus ELISA on serum that detects IgM because the assay detects an ACUTE response to the virus
what diagnostic test would you select for a 2 yo ram showing signs of infertility and unilateral epididymal enlargement
brucella ovis ELISA on serum
what diagnostic test would you select for a pleural fluid sample from a 3 yo indoor/outdoor cat with lethargy, inappetence, dyspnea, tachypnea, and lung retraction on rads
aerobic and anaerobic culture on pleural fluid
(expecting nocardia)
